The invention relates to the preparation of a water-dilutable paint capable of protecting a painted surface against stone chips and compensation primer, to their use and to processes for producing the same, especially in the motorcar industry.
A basic problem in the lacquering of surfaces, particularly surfaces of automobile bodies, is the resistance of the overall lacquer to chipping by stones. The stone impact resistance of conventional paints depends substantially on the surface primer layer located on the electrocoat layer. The surface primer layer, having a thickness of about 30 to 40 xcexcm, smoothes the rough surface of the crude automobile body to allow subsequent coating with a covering lacquer, fills up minor surface inequalities, and mainly offers protection against mechanical attacks (stone chip protection function).
A conventional automobile lacquer coating according to the so-called xe2x80x9cbase coat/clear coat processxe2x80x9d consists, for example, of a total of four layers. These four layers are applied one after the other in separate lacquering devices. The first layer, directly located on the car sheet metal, is the electrocoat layer which is applied by electro-dipcoatingxe2x80x94mainly cathodic dipcoating (CDC)xe2x80x94for protection against corrosion. The nature of the electro-dipcoating prevents the formation of overspray.
The second layer, located on the electrocoat layer and having a layer thickness of about 30 to 40 xcexcm is the so-called surface primer layer which, on the one hand, smoothes the rough surface of the crude automobile body to allow subsequent coating with a covering lacquer, it also levels minor surface inequalities and mainly offers protection against mechanical attacks (stone chip protection function). This layer is substantially produced by electrostatic application of a baking enamel, for example, with electrostatic high rotation bell, followed by baking at temperatures of above 160xc2x0 C.
The third layer, located on the surfacer primer layer, is the base coat layer which imparts to the automobile body the desired color by virtue of adequate pigments. The water soluble base coat is applied by conventional spray methods.
The fourth and uppermost layer, located on the base coat layer, is the clear coat layer which, analogous to the base coat layer, is applied by conventional spray methods and, which on the one hand, imparts the desired lustre and, on the other hand, protects the base coat against environmental factors (UV radiation, salt water etc.).
The object of the invention is to produce a paint which protects against stone chip which meets the requirements of the tests prescribed by the automobile industry and, at the same time, takes over the function of the conventional surface primer layer or the function of the conventional surface primer layer and the conventional base coat layer.
This problem is solved in the present invention by providing a water-dilutable paint capable of protecting the painted surface against stone chips (hereinafter referred to as xe2x80x9cstone chip protection paintxe2x80x9d), containing from 20 to 60% by weight binders and from 2 to 10% by weight pigments, and a water-dilutable compensation primer, containing from 40 to 80% by weight binders. The compensation primer may additionally contain up to 20% by weight pigments.
Pigments, as defined by the present invention, include inorganic or organic multicoloured or achromatic colorants that are practically insoluble in the medium in which they are applied, such as described, for example, in xe2x80x9cGlasurit-Handbuch Lacke und Farbenxe2x80x9d, 11th edition, published by Curt R. Vincentz, Hannover 1984, p. 97-108, i.e. carbon black titanium dioxide, effect pigments, such as aluminum bronzes, mica pigments and the like.
A binder as defined here and in the text following includes substances which bond together similar or different kinds of substances, in particular the non-volatile component of a lacquer without pigment and filler, but including plasticizers, drying agents and other non-volatile additives, preferably the water compatible film-forming resins, such as polyester, polyurethane and acrylate resins and the like, as described, for example, in xe2x80x9cGlasurit-Handbuch Lacke und Farbenxe2x80x9d, loc. cit. p. 19-96 or in H. Wagner, H. F. Sarx xe2x80x9cLackkunstharz exe2x80x9d, Carl Hanser Verlag Munich 1972.
In a further embodiment of the invention, the ratio of binders to pigments in the water-dilutable stone chip protection paint is between 5:1 and 12:1 or between 10:1 and 15:1 in the case of the water-dilutable compensation primer.
Inasmuch as the water-dilutable stone chip protection paint or the compensation primer contains pigments, the latter can be an effect pigment, in particular an aluminium bronze.
In a preferred embodiment of the invention, the binder of the stone chip protection primer or the compensation paint is a water compatible blocked isocyanate, a polyurethane resin, a polyester resin and/or melamine resin.
In a further embodiment of the present invention, the polyester resin has of the present invention a molecular weight of from 5,000 to 10,000; exhibits an adequate number of carboxyl groups which after neutralization with bases impart to the polyester resin sufficient water-dilutable properties; and contains functional groups, in particular hydroxyl groups which render the polyester resin cross-linkable.
The polyester resin can have an OH number of from 20 to 80, in particular from 30 to 60 and an acid number between 10 and 50, in particular from 15 to 35.
Its glass transition temperature (Tg) may be between xe2x88x9220 and +30. In a further embodiment of the invention, the polyester resin is in particular a polycondensation product of a diol and a dicarboxylic acid in the presence of a component containing more than 2 functional groups.
The diol is selected from the group consisting of 1,6-hexanediol, neopentyl glycol, 1,4-dimethylolcyclohexane, hydroxypivalic acid neopentyl glycol ester (HPN), perhydrogenated bisphenol A, trimethylolpropane and trimethylolpropane monoallyl ether.
The dicarboxylic acid is selected from the group consisting of adipic acid, phthalic acid, isophthalic acid, hexahydrophthalic acid, tetrahydrophthalic acid or their possible anhydrides, in particular from the group of dimeric fatty acids.
The component containing more than 2 functional groups may be a triol, a tricarboxylic acid, a monohydroxydicarboxylic acid, in particular a dihydroxymonocarboxylic acid, preferably trimellitic acid, trimethylolpropane and dimethylolpropionic acid.
The best properties can be achieved with binders exhibiting elastomeric characteristics after baking. These can be specific polyurethanes in which, due to their segmented structure, a specific sequence of soft and hard segments is observed. Preferred binders are soft polyester resins having a high molecular weight and a high hydroxy-functionality, obtained by polycondensation of dimeric fatty acids and polyalcohols, preferably diols, their crosslinked sites being produced by trifunctional (hydroxy)carboxylic acids. These polyester resins are also used with blocked polyisocyanates or melamine resins as cross-linking agents.
Furthermore, the water-dilutable stone impact-protection paint or compensation primer can additionally contain organic solvents and additives.
The term solvent as used hereinafter includes those organic substances which can dissolve other substances in a physical manner, such as, e.g., lower alcohols, glycol ether, lower ketones, in particular organic solvents which are substantially miscible with water, such as butanol, isopropanol, methylethyl ketone and many others, such as described for example, in xe2x80x9cGlasurit-Handbuch Lacke und Farbenxe2x80x9d, loc. cit. pages 117-138.
Additives are defined as substances that are added in small amounts to other substances, in particular liquid substances, to change their properties in a desired manner or to facilitate their processing. Additives include gloss products, wetting agents, drying agents sedimentation inhibitors, antifloating agents, film-inhibiting agents, leveling agents, release agents, lubricants as well as UV absorbers, biocides, plasticizers, antistats, stabilizers, antioxidants, antiozonants, fillers, viscosity control agents, aging inhibitors, detergents, dispersing agents, defoamers, setting accelerators, solidification retarders or drying agents, as described, e.g., in xe2x80x9cGlasurit-Handbuch Lacke und Farbenxe2x80x9d, loc. cit. p. 113-117.
The invention also relates to the use of a water-dilutable stone chip protection paint for coating automobile bodies having an electro-dip primer coat, a second so-called surface primer layer, a third layer comprising a water-soluble base coat, and a fourth layer comprising a covering layer of clear coat, the surface primer layer being replaced by a stone chip protection layer consisting of the stone chip protection paint and having a thickness of from 10 to 20 xcexcm.
In a further embodiment of the present invention, the water-dilutable stone chip protection paint is used for coating automobile bodies having an electro-dip primer coat, a stone chip protection layer consisting of the stone chip protection paint and having a thickness of from 10 to 20 xcexcm, and a covering layer of clear coat.
The water-dilutable compensation primer of the present invention is used in admixture with a commercial base coat for coating automobile bodies having thereon an electro-dip primer coat, a second so-called surface primer layer, a third layer consisting of a water-soluble base coat and a fourth layer comprising a surface primer covering layer of clear coat, the surface primer layer being replaced by a mixture of water-dilutable compensation primer and a commercial base coat and having a thickness of from 10 to 20 xcexcm.
According to another embodiment of the invention, the water-dilutable compensation primer is used in admixture with a commercial base coat for coating automobile bodies having an electro-dip primer coat, a layer comprising the mixture of water-dilutable compensation primer and commercial base coat and having a thickness of from 10 to 20 xcexcm and a covering layer of clear coat.
Thus, the compensation primer serves to condition the base coat, i.e. it imparts to a conventional base coat stone chip protection and surface primer properties so that the conditioned base coat either replaces only the surface primer layer (four layer-structure); or the surface primer layer and the base coat layer simultaneously (three layer-structure). This conditioning secures in particular the adhesion of the stone chip protection layer in such a manner that if a strong mechanical impact from an outward source acts on the overall lacquering, which if using a three layer-coating (i.e. without surface primer layer) without the layer of the present invention the impact would lead to a chipping off of the electrocoat layer. The adherence to the electrocoat layer is controlled to such an extent that, on the one hand, the stone chip protection layer is released from the electrocoat layer, but does not pull the latter from the automobile sheet metal and, on the other hand, the chip marks are as little as possible. The latter can be achieved by additives which specifically increase the rebound resilience of the stone chip protection paint. Rebound resistance as used herein means the property by means of which a mechanical impulse (stone impact) is counterbalanced by an elastic deformation. Damage of the material is inhibited thereby.
There are different methods of testing the property for predetermined stability of the pertinent layer, namely of said stone impact protection primer or of a base coat conditioned by said compensation paint, said methods being adapted to the specific field of application; however, all of the tests aim at reproducing the mechanical action as exactly as possible. There are, for example, prescribed tests in the automobile industry which try to simulate the stone impact by the impact of a shot (Mercedes-Benz AG, lacquer testing apparatus with shot testing apparatus at temperatures of xe2x88x9220 to +50xc2x0 C., a shot diameter of from 2 to 4 mm and a shot speed of from 50 to 300 km/h) or a chisel-like testing device (Test VDA 621-428 of BMW-AG) on the finished lacquered surface at exactly defined temperatures (room temperature and xe2x88x9220xc2x0 C.) and further exactly defined parameters.
In a particularly preferred embodiment of the invention, the water-dilutable stone impact protection paint contains overspray from water-soluble base coats, in particular from spraybooth waste water.
An overspray is the excess of lacquer which during the application of the lacquer by spray painting techniques does not hit the article being lacquered but misses the article and is discharged together with the exhaust air from the lacquering zone in order to be transferred, for example in a wet wash discharge step, to the wash discharge water of the sprayboothsxe2x80x94referred to hereinafter as booth waste waterxe2x80x94to wash discharge the lacquer components present in the overspray.
A big problem generally encountered in lacquering processes is the waste material, in particular paint sludges. Paint sludges are usually produced when the substantially water-insoluble lacquer components are removed from the booth waste water by coagulation to allow the booth waste water, recovered after sedimentation or separation of the paint sludge, to be recycled for reasons of environmental protection.
In view of the high costs of waste disposal, the lacquer manufacturing and the lacquer processing industries are increasingly interested in applying new methods to avoid this kind of waste. To this end, there are basically two possibilities. On the one hand, one is trying to practically inhibit the formation of an overspray right from the beginning by applying specific and more sophisticated lacquering methods. The latter include flow coating, casting, dipcoating, roll coating, coil coating, electro-dipcoating or the like. On the other hand, one is trying to recycle the overspray which is necessarily always formed, for example, when using spray painting techniques. At present, the degree of coating efficiency of this spraying method (air, airless or air mix atomization etc.) is from 20 to 80%, i.e. only from 20 to 80% of the sprayed lacquer hit the surface of the article being lacquered, the rest accrues as overspray.
A conventional water-soluble base coat usually consists of
from 5 to 15% by weight pigments,
from 10 to 20% by weight binders
0 to 20% by weight organic solvents
0 to 5% by weight additives
40 to 85% by weight water.
A particularly serious problem is the recycling of base coat oversprays. Since, for logistic reasons, the automobile bodies in a factory are not sprayed serially with the different colors (i.e. not only black on Mondays, only white on Tuesday and only red bodies on Wednesdays etc.) but several color shades are sprayed within one hour. One always obtains a variable amount of water-soluble base coat overspray of different colors in the booth waste water. Therefore, the booth waste water has an unsightly shot color from sludgy brown to dirty gray. This excludes the return of this component to the base coat being coated, as proposed in DE-OS 42 13 671.
To date, in all automobile body lacquering plants, the overspray booth waste waters containing is converted into a lacquer sludge by coagulating agents. In former times, this lacquer sludge was transported to a waste dump, which is now forbidden by law. Therefore, nowadays this coagulate or paint sludge, which still contains about 50% by weight water, is reprocessed, after removal of the main amount of water, drying in rotating tubular kilns and grinding to form granulates or powder, and is reutilized for other purposes. This means in the case of the granulates, that they are disposed of by the so-called xe2x80x9cthermal recoveryxe2x80x9d, i.e. incineration of special waste. The energy recovered thereby does neither compensate the loss of material nor the expenditure of method and capital.
The recovery of the booth waste water-overspray in the sense of a recycling to the spraying process, i.e. a recovery on a higher level, is only possible in the processing of water thinnable lacquers. Water thinnable lacquers are those systems which contain, besides the conventional lacquer components, water-dispersible binders and which have water as the main solvent. When using water thinnable lacquers in spray coating techniques, one has the possibility of precipitating the overspray in the booth waste water in such a manner that coagulation does not take place. Thus, the booth waste water becomes a very diluted water thinnable lacquer which can be extracted from excess water by suitable methods and be reutilized as lacquer. Thus, DE-OS 42 13 671 describes a process for recovering the overspray from aqueous coating compositions during spray coating in water dripping spraybooths by means of ultrafiltration which allows to concentrate the enriched booth waste water (retentate) to a solids content of up to 35% by weight. DE-OS 42 07 425 describes a process for recovering the lacquer overspray of aqueous lacquers during spray coating by ultrafiltration and subsequent electrophoresis. Further processes for recovering water thinnable lacquer oversprays are described in DE-OS 34 28 300, CH-OS 1656/59, DE-OS 41 33 130 and DE-OS 42 02 539.
It has also been contemplated to recycle the booth waste water until a water thinnable lacquer concentration is obtained in said booth waste water which permits the direct use of the latter as thinnable lacquer. JP-PS 49 51 324 teaches to increase the concentration of lacquer in the lacquer-containing precipitate by removal of water from the entire booth waste water to such an extent that this precipitate can be reutilized for color spray painting. However, practical tests showed that a concentration of thinnable lacquers in booth waste water to a solids concentration of more than 2% by weight results in that the overspray is no longer completely washed out and the discharged exhaust air contains more overspray than is prescribed by the law (technical instructions on air quality control). The solids concentration refers to the overall amount of non-volatile lacquer components, such as pigments, binders, additives etc., expressed as weight percent, based on the total content of all lacquer components.
DE-OS 42 13 671 and 41 33 130 teach methods of recovering the oversprays of aqueous coating compositions during spray coating in spray booths according to which part of the liquid circulating in the ultrafiltration cycle is used as aqueous coating composition for spray coating. This circulating liquid is always added to the same spraying composition from which overspray it is obtained.
The overspray comprising aqueous base suitable for producing a stone impact protection layer which dries at from 50 to 90xc2x0 C. is used according to the invention between the electrocoat layer and the base lacquer layer.
For the purposes of the invention, this overspray can be recovered by any conventional method. Its accumulation can be effected, for example, according to the lacquer recovery methods described in I-Lack 61 (1993), p. 425-428 (washer device, recovery wall, recovery belt, Relas lamella-recovery system). The overspray is preferably by means of any prior art wet wash-out method in a condition allowing its use according to the present invention. This wet wash out results in an accumulation of the overspray in the booth waste water.
With respect to the composition of the individual lacquer components, the overspray in the booth waste water has a different composition from the base coat since, for example, the evaporation of the volatile organic solvents is quicker and, they, escape to the atmosphere together with the exhaust air from the spraybooth.
The booth waste water normally consists of
form 0.1 to 1.5% by weight pigments,
from 0.1 to 2.0% by weight binders,
from 0 to 2.0% by weight organic solvents,
from 0 to 0.5% by weight additives and
from 94 to 99.8% by weight water.
Booth waste water containing an overspray which consists of water-soluble base coats of different colors is preferably used for the purposes of the invention.
This booth waste water is preferably used in a water-depleted upgraded form in the light of the invention. This upgrading by water depletion can be effected by any separation procedure familiar to the expert, such as concentration by vaporization, concentration by boiling in vacuo, freezing out, centrifugation or freeze drying. An upgrading of the booth waste water is usually conducted by ultrafiltration or a combination of ultrafiltration and electrophoresis (DE-OS 42 97 425).
This upgraded booth waste water has a solids concentration of from 10 to 35% by weight. In upgrading the overspray absorbed in water, there are changes with respect to the composition of the concentrate because, for example, preferably water-insoluble and high molecular weight components such as pigments and binders are retained by ultrafiltration while the water soluble and low molecular weight components, such as solvents, melamine resins and neutralizing agents preferably are transferred to the permeate. The water depletion can also result in a change of the properties of the paint components, such as color shade and rheology.
The upgraded booth waste water usually consists of
from 2 to 12% by weight pigments,
from 4 to 18% by weight binders,
from 0 to 5% by weight organic solvents,
from 0 to 0.8% by weight additives,
from 64 to 94% by weight water.
Thus, by virtue of the production of the stone chip protection paint of the present invention, a big problem of the spray coating technique according to the xe2x80x9cbase coat/clear coatxe2x80x9d method is solved because all of the overspray accrued in the spray coating of the base coat layer in the booth waste water can be utilized. This means that it is no longer necessary to coagulate booth waste water and, as a result, paint sludge is not obtained when the base coat layer is applied.
In addition, this means that both the base coat overspray as well as the overspray of the stone chip protection paint recovered therefrom can be blended in one booth waste water.
A basic advantage of using booth waste water for producing stone chip protection paint is that although the booth waste water may contain different color shades of the water-soluble base coats, it can, nevertheless, be reutilized as valuable material.
A further particular advantage in replacing the conventional filler layer by the stone chip protection layer of the presents invention consists in the fact that the baking of this surface primer (filler) layer can be dispensed with. The stone chip protection layer of the present invention dries at a temperature of between 50 and 90xc2x0 C. and, consequently, necessitates, for example, only a drying or a predrying with infrared radiation. This means a considerable saving of energy and time. Moreover, it is not necessary to apply the stone chip protection layer of the present invention with the same layer thickness as the surface primer layer; it is enough if said stone chip protection layer has a thickness between 10 and 20 xcexcm or sufficient to possess the same mechanical properties as the conventional surface primer layer. This also means a considerable saving of material.
A further advantage over the prior art is only two instead of three booth waste water cycles have to be installed. The recycling of the clear coat, e.g. by ultrafiltration, makes it possible to run for the first time an automobile standard processing which is completely free of paint sludge. When water-borne clear coats are used, it is even possible to run the process with only one booth waste water cycle if the coating efficiency of the water borne clear coat application is optimized by suitable methods such that the accrued amounts of overspray do not exceed the requisite amount of the stone chip protection paint of the present invention. Besides, the requisite amount of stone chip protection paint can be controlled by varying the layer thickness, (in particular heightening).
In case the stone chip protection paint, manufactured according to the invention from the concentrated booth waste water, does not exhibit the desired protective properties against mechanical load acting on the overall lacquering from outside, it can be conditioned as specified in the invention.
This conditioning is effected by adding the components necessary for the desired properties of the stone chip protection layer. It may also be advantageous to remove the effect pigments, partly or completely.
The object of this conditioning is, in particular, to adjust the adhesion of the stone impact protection layer in such a manner that if a strong mechanical load acts on the overall lacquering from outside, which without the stone chip protection paint of thee invention and the use of a three-layer lacquering (i.e. without surface primer layer) would entail a chipping off of the electrocoat, the adhesion to the electrocoat layer is adapted in such a manner that, on the one hand, the stone chip protection layer is released from the electrocoat layer but the latter is not torn off therewith from the automobile body and, on the other hand, the chips on the paint surface are as small as possible. The latter can be achieved by additives which increase the rebound resilience of the stone chip protection paint to a predetermined degree. The term rebound resilience stands for the property by means of which a mechanical impulse (stone impact) is compensated by an elastic deformation. Damage of the material is thus inhibited.
The above-mentioned mechanical loads acting on the overall lacquering from outside may involve any kind of mechanical action, such as, for example, stone impact on automobile lacquerings, frictional contact or impact contact between lacquered articles or any kind of objects on laquered surfaces.
For conditioning the mechanical properties of the stone chip protection paint of the present invention, binders, such as a water compatible blocked isocyanate, a melamine resin, a polyurethane resin or polyester resin can be used; organic solvents, pigments or additives can also be used as suitable components.
These components can be added to the stone chip protection paint alone or in any combination or also as compensation paint having a solids concentration of from 20 to 80% by weight.
This compensation paint contains all or parts of the components necessary for conditioning the desired mechanical properties of the stone impact-protective layer.
Thus, a stone chip protection layer cycle is formed according to the invention, to which base coat overspray and compensation primer are periodically added.
Subsequent to the conditioning of the upgraded booth waste water, an equilibrium is established after some time within said cycle which can be maintained by the appropriate amount of compensation paint, which can be easily determined by the expert.
The particular advantage in the use of the compensation paint resides in that all components can be added to the stone chip protection paint, adapted to the pertinent base coat system, to the corresponding overspray or to the booth waste water, and that in a single mixing procedure directly in the spray coating plant or its direct neighborhood. Therefore, the concentrated booth waste water, which can contain up to 90% water, must not be transported from the lacquer processing plant to another place where it is tested in respect of the properties required for the stone chip protection layer and, if desired, conditioned by the method of the present invention, but all these procedures can be done in the same place where the lacquering is conducted.
The higher the concentration of the compensation paint, the less water must be removed from the booth waste water. The process of lacquering automobile bodies comprises applying an electro-dipcoat primer (CDC), baking the same, applying the stone chip protection paint of the present invention, drying the same at 50-90xc2x0 C., applying a base lacquer, drying the same at 50-100xc2x0 C., applying a clear coat and subsequently jointly baking the three lacquer layers at 130-160xc2x0 C.
The stone chip protection paint of the present invention can be used for lacquering all solid, planar or non-planar, smooth or uneven metallic surfaces which are coated according to the above-specified xe2x80x9cbase coat/clear coat processxe2x80x9d and which are to be protected against mechanical load outwardly acting on the overall lacquer, said protection paint being located between the electrocoat layer and the base coat layer. Smooth metallic surfaces, such as occur in the lacquering of plants, apparatuses, windows, in particular motor vehicles (cars, trucks, bikes etc.) are mainly involved.